A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to control the lithographic process to place device features accurately on the substrate, alignment marks are generally provided on the substrate, and the lithographic apparatus includes one or more alignment sensors by which positions of marks on a substrate should be measured accurately. These alignment sensors are effectively position measuring apparatuses. Different types of marks and different types of alignment sensors are known from different times and different manufacturers. A type of sensor widely used in current lithographic apparatus is based on a self-referencing interferometer as described in U.S. Pat. No. 6,961,116 (den Boef et al). Generally marks are measured separately to obtain X- and Y-positions. A combined X- and Y-measurement can be performed using the techniques described in published patent application US 2009/195768 A (Bijnen et al), however. The contents of both of these applications are incorporated herein by reference.
There is continually a need to provide more accurate position measurements, especially to control overlay error as product features get smaller and smaller. For this purpose, it is considered to increase the numerical aperture of the alignment sensors. However, by doing this the sensors become more sensitive to defocus and local tilt of the substrate, which can lead to an apparent position deviation (APD) and color to color variations when radiation with multiple wavelengths are used. The separate level sensors currently used to move the substrate into focus of the alignment sensors are not able to detect the defocus and local tilt of the substrate to such an extent that the increased sensitivity can be compensated for, because the alignment marks are usually buried underneath a relatively thick stack of layers, while the level sensors are designed to probe the top of the stack of layers.